We don't know really. The best sources we have is probably gildas because he is closest timewise:
What that actually means is up to debate, but the most likely answer is that his father was the governor of Maxima Caesariensis who was of consular rank and thus could wear purple. So in the event of Victor being forced to flee to Britain, Ambrosius' family is likely already an important one in Britain and thus will be close to Victor's imperial court. Close enough perhaps for Ambrosius or his father to marry into the imperial family.
That sounds really intresting, is there a good resource to see the family tree of these important families ? Either Victor could marry into ambrosius family if ambrosius had a sister or ambrosius marrying the doughter of victor. Do you think with victor fleeing to britannia, there would still be a constanstin III rising ?